On the 9-inch there are 31 spline and 28 spline axles. You'll find the 31's in vans and trucks mostly. Also there are some earlier 9-inch's which have a housing without the big hump, and it has two dimples. It almost looks like an 8-inch, but the 3rd member is more rounded and not flat on the top and bottom. I figured out the width of rearend I needed by the wheels I want (3.5" backspacing) and got one out of a '65 thunderbird. It was a 2-dimple 2.70 something, 28 spline. I found that the ring gear is actually 9 3/8? so it is an odd size, but I have another 3rd member I'm going to swap in it with 3.55 gears as soon as I change it to 28 spline.

'53 Chevy truck 1/2 ton